Silver halide is reduced to silver metal in developing solutions used in the processing of photographic materials, the reaction being catalysed by clusters of silver atoms which form on the silver halide grains in the emulsion during exposure to light. In general, it is necessary to carry out a process of chemical sensitisation prior to coating the emulsion to form preferential sites on the grains for the latent imaging process to occur. Chemical sensitisation is particularly important for emulsions of high sensitivity having larger silver halide grains, e.g., those of mean volume exceeding 0.1 cubic microns which have many competing sites at defects within the grains where photoelectrons derived on exposure may otherwise be dissipated without contributing to formation of silver specks useful for catalysing development.
Chemical sensitisation usually takes place after both completion of the growth of the silver halide grains in gelatine and removal of ionic by-products. Generally, the emulsion is sensitised by the addition of small molecule sulphide-releasing compounds, for example, sodium thiosulphate, usually in the presence of gold compounds.
During a period of maturation at an elevated temperature, the emulsion progressively undergoes a larger increase in sensitivity to light. This sensitivity increase is due to formation on the grains of small deposits of silver sulphide, or mixed silver/gold sulphides, which act as the required preferential sites for trapping of photoelectrons and their reaction with interstitial silver ions to assemble the catalytic Ag.degree. clusters. The photoelectrons may be generated by exposure to blue or UV light, which the silver halide is naturally able to capture, or to light of longer wavelength captured using sensitising dyes additionally adsorbed to the grain surface.
A basic problem in practising chemical sensitisation is the formation of excess silver sulphide on the grain surface during the course of the digestion process used to create centres conferring maximum sensitivity on the majority of grains. The factors controlling deposition of silver sulphide are; quantity and type of sulphide releasing compound added; bulk Ag.sup.+ and H.sup.+ concentrations in the emulsions; reaction temperature; duration of heating; and adsorption of additional compounds to the grain surface.
Depending on the choice of these variables, sensitisation of the emulsion may be accompanied by the formation of fogged (spontaneously developable) grains; the formation of competing sensitivity speck sites, resulting in fragmented latent image, so that a higher light exposure is needed to enable development; and the formation of diffuse Ag.sub.2 S that does not influence latent image formation as such, but confers unwanted red light sensitivity to the emulsion, due to the red adsorption band of Ag.sub.2 S.
By avoiding the use of high Ag.sup.+ ion concentration or high pH, limiting the amount of S-sensitiser, and stopping sensitisation by thermal quenching after an optimum duration, sulphide deposition can normally be controlled so as to proceed to a point of giving maximum sensitivity consistent with a just perceptible rise in optical density in non light struck areas (DMIN) due to spontaneous developability. By extending or shortening the heating period it is possible to obtain an acceptable level of sensitivity at low DMIN from a range of different amounts of a given sulphur sensitiser such as thiosulphate, even under conditions where the sensitiser reacts rapidly with the AgX surface so that a largely complete transfer of sulphide takes place. The excess Ag.sub.2 S generated by higher sensitiser amounts is present either at the competing site, or diffuse forms described above. Either of these cases gives rise to an increased sensitivity to red light. For many photographic materials which are not intended for red light exposure, this red light sensitivity is a disadvantage in itself as it limits the amount of safelight which can be tolerated, and it is desirable to minimise it. Because of the ratio in which S-sensitizers partition between grains of different size and surface topography in an emulsion, it is necessary to accept a substantial degree of oversensitisation of this sort to ensure that the majority of the emulsion can reach a high degree of sensitivity.
There are a number of disclosures relating to development accelerators and reduction sensitisation where sulphur containing compounds are used. However, chemical sensitisation is referred to as a separate process and where chemical sensitisation is described, the process is generally carried out by the conventional methods, adding sodium thiosulphate to the silver halide emulsion.
The use of polymeric thioethers as development accelerators, as an alternative to alkylene oxide polymers, is described in U.S. Pat. Nos. 3,779,769 and 3,813,247, which both employ thioether copolymers containing the --CH.sub.2 --CH.sub.2 --S--CH.sub.2 --CH.sub.2 -- moiety. A clear distinction between actual chemical sensitisers and these development accelerators, also referred to as "speed addenda", is taught in these patents. Whereas chemical sensitisers result in the formation of silver sulphide on the surface of the silver halide crystal, development accelerators increase sensitivity without apparently entering into chemical combination with the silver halide. They increase speed by their presence during exposure and processing and require no digestion with the photographic emulsion to produce the increase in speed. Similar use of polymeric thioethers as development accelerators is described in U.S. Pat. No. 3,615,500, using polymers containing a thiomorpholine unit, and respectively using esters (--O--CH.sub.2 --CH.sub.2 --S--CH.sub.2 --CH.sub.2 --OCO(CH.sub.2).sub.3 CO) in U.S. Pat. No. 5,041,367, and R--S--(R'O).sub.n -- in JP70010989. Alternatively, similar polymeric thioethers are placed in the developing solution in JP3144440. Polymers having thioether-containing pendant groups have been used as synthetic peptisers in JP4151140, and in JP4235546 as deflocculants. In all these cases, the sulphur atoms in the polymers do not form part of functional groups which are known to be labile S-releasers (cf. thiocarbonyl or polysulphide groups) and addition of these polymers in the absence of other, labile sulphur compounds would not be expected to cause S-sensitisation. Consequently a separate chemical digestion stage using conventional sensitisers is provided in these patents for sulphur-sensitisation of the emulsions.
Reduction sensitisation of emulsions in the presence of R--O.sub.2-- SM, R--SO.sub.2 --SR, or R--SO.sub.2 --S--L.sub.m --SSO.sub.2 R' is described in U.S. Pat. No. 5,254,456, U.S. Pat. No. 5,079,138 and U.S. Pat. No. 5,061,614 using ascorbic acid, and in EP348934 using thiourea dioxide, dimethylaminoborane or stannous chloride as the sensitising reductant. In these patents the sulphur containing polymers function only as an auxiliary to the process of reduction sensitisation, the purpose of which is to improve efficiency of photoelectron utilisation by providing Ag.sub.2 .degree. centres inside the AgX grains to counter electron-hole recombination. After this reduction sensitisation it is still necessary to carry out surface sulphur-sensitisation to provide sites for latent image formation, and in these patents this is done using the conventional S-sensitiser, sodium thiosulphate, in combination with chloroaurate. The addition of polymer having a pendant ArS.sub.2 O.sup.2- group to a high sensitivity emulsion before coating has also been described in JP1079742, resulting in improved storage stability. The above cited U.S. Pat. No. 3,779,769 and also SU863595 describe addition to photographic emulsions of polymers containing sulphonate groups RSO.sub.3 Na or RSO.sub.3 NH.sub.4, another form in which sulphur has no sensitising activity.
Sensitisation of simple AgBr emulsions by mixing with high Mw compounds described as having the structure [AgS(R)].sub.n, prepared by adding silver nitrate to the thiols cysteine, 2-aminoethanethiol, or 2-mercaptoethanol are described in BE 767486. These compounds, for which the structures described are inorganic ionic aggregates, consisting of linked sulphide and silver ions, Ag.sup.+ . . . (R)S.sup.- . . . Ag.sup.+ . . . , are outside the scope of conventional organic covalently-bonded-backbone polymers.
It has long been known that certain types of gelatin apparently exert a chemical sensitising effect on silver halide emulsions, but it is now well established that this is caused by low molecular weight impurities in the gelatin, and is not caused by functional groups attached to the gelatin polymer itself (see, for example, p.263 of "Photographic Materials and Processes" (Stroebel, Compton, Current and Zakia), pub. Focal Press, 1986; and also "Photographic Gelatin" (S. E. Sheppard), Photographic Journal, August 1925, pp 380-387). Modern photographic gelatins are normally free of such impurities.